Lightweight panels may be made by bonding face sheets to an inner core. The face sheets may be of a woven fabric and the core may be of honeycomb, corrugated material or a plurality of ribs. The composite may be dipped or sprayed with a plastic material to give it strength and rigidity. The fabric may be passed through a resin solution in a technique known as preimpregnation wherein the amount of resin remaining on the fibers is controlled. Thereafter, mandrels are fitted into the panel cells and the panel is cured, and made rigid, under appropriate heat and pressure in a suitably formed mold. After removal of the mandrels, additional sound attenuating materials may be placed in the cavities, if desired.
In the foregoing bonded panels the problem always exists that delamination may occur due to resin or adhesive bond failures. One method of overcoming this problem is to weave the top, bottom and core as an integral unit such as shown in U.S. Pat. No. 2,607,104 for Corrugated Fabric and Method of Making the Same, issuing to B. H. Foster on Aug. 19, 1952 and assigned to U.S. Rubber Company. In this case the upper and lower plies are so woven with shrinkable fabric that, after heat is applied, these plies shrink, leaving the center ply as a corrugation. Insulation then fills the area between the outer plies. While this teaching serves the purpose intended, the weave of the outer ply cannot be oriented for optimum strength and acoustic purposes. This same objection applies to the three-dimensional weaving disclosed in U.S. Pat. No. 3,481,427 for Acoustic Panel Structure issuing to Dobbs and Holmes on Dec. 2, 1969 and assigned to the present assignee. U.S. Pat. No. 3,769.142 for Non-Woven Locked Ply Composite Structure issued Oct. 30, 1973 to Holmes and Dobbs and is assigned to the present assignee. In this patent multiple layers of non-prebuckled fibrous material are angularly oriented and interlocked with vertical fiber locks to prevent interlaminar shear. The angular orientation of the plies provide strength in desired directions and also effect the face sheet openings for the acoustical purpose of sound suppression. While this structure is excellent for its intended purpose, further development is needed in apparatus for fabricating such structure.